a. Field
The instant invention relates to stacked switched capacitor (SSC) energy buffers.
b. Background
Power conversion systems that interface between direct current (DC) and single-phase alternating current (AC) require an energy storage capability (or an energy buffer) that provides buffering between a constant power desired by a DC source or a load and a continuously varying power desired for a single-phase AC system.
The flow to and from such an energy buffer is at twice the line frequency (e.g., 120 Hz in the United States). The buffering energy requirement can be calculated as P/ωline, where P is the system average power and ωline is the line angular frequency. Because the energy storage requirement of the buffer is proportional to the system average power and the (relatively long) line period (Tline=2π/ωline), the size of the required energy buffer cannot be reduced simply through increases in switching frequency of an interface power converter. Thus, energy buffering requirements represent a significant limitation on miniaturization of grid interface systems.
One consideration associated with twice-line-frequency energy buffering relates to lifetime and reliability. Conventional power conversion systems typically utilize electrolytic capacitors to provide high-density energy storage for buffering. It is, however, widely appreciated that despite providing the best available energy density and providing small DC bus voltage variation, electrolytic capacitors also represent a significant source of system lifetime and reliability problems. Also, electrolytic capacitors can only be operated over a narrow charge/discharge range at 120 Hz for thermal and efficiency reasons (i.e., associated with RMS current limits and efficiency requirements). These considerations directly limit the energy buffering capability of electrolytic capacitors at 120 Hz. Thus, while typical peak energy storage densities of up to 0.9 J/cm3 can be achieved with electrolytic capacitors, the allowable energy swing at 120 Hz yields practical energy densities that are about an order of magnitude lower. Hence, the development of energy buffering circuits that eliminate electrolytic capacitors while maintaining high energy storage density and high efficiency is one important requirement to achieving future grid interface systems that have both a small size and a high reliability.
Film and ceramic capacitors have much longer lifetime, but lower energy density. To compensate for their lower energy density, film and ceramic capacitors can be charged and discharged over a wider voltage range than is practical with electrolytic capacitors at relatively high frequencies, provided a mechanism is available to maintain the dc bus voltage within a required narrow range. A number of strategies to increase the energy utilization of capacitors have been proposed, including the use of an additional bidirectional dc-dc converter, an energy buffer incorporated into the power stage and switched capacitor energy buffers.